JP2758224B2 - Optical storage device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical storage device capable of extremely high-speed access using light.

<Prior Art> Conventionally, as an optical storage device, a method of directly recording information like a microfish technology based on photographic technology,
Alternatively, various types of apparatuses have been proposed, such as a method of storing a diffraction image as a Fourier transform image in the form of a hologram. Among them, an optical disk device that uses a laser beam for storage and reading (reproduction) is particularly noted as a device for communication and information processing. Examples of this type of device include a device widely used as a consumer device such as a CD (compact disk) or a laser disk, and a device being developed as a large-capacity storage medium for a large computer. . The basic operation principle is as follows: 1) Irradiating a laser beam to a specific portion on a disk causes a spot having a reflectance different from that of the surroundings due to optical damage, optical structural change, magnetic transition, and the like, and this is used as information.
Information can be stored as spots, spot densities, and the like.

2) A laser beam is irradiated to a position where a spot is (or should be), and information detection, that is, a read operation is performed based on a change in intensity of a reflected wave.

3) Scanning of the space where the information corresponding to the address exists,
This is performed by rotating the disk and mechanically scanning the head in the radial direction.

4) In the case of a storage method based on an operation involving an irreversible structural change, the storage device is a read-only storage device, but can be used as a rewritable device by utilizing an operation such as magnetic transition.

<Problem to be Solved by the Invention> Such a conventional optical storage device is more
It has excellent features such as large capacity, simplified peripheral circuits, and long life due to non-contact. However, as described above, access to information involves a mechanical operation, so that only extremely slow operation can be expected. That is, the conventional optical storage device is effective as a secondary storage medium, but cannot be used as a storage device requiring high-speed access as represented by a semiconductor cache memory.

<Object of the Invention> An object of the present invention is to realize an optical storage device that does not require mechanical scanning as in a conventional optical storage device and that can be accessed at an ultra-high speed. In particular, a configuration utilizing coherent light phase information is intended.

<Means for Solving the Problems> An optical storage device according to the present invention for solving the above-mentioned problems includes: a) an optical fiber or a planar optical waveguide as a medium for storing information and capable of propagating an optical pulse. B) a plurality of reflection media capable of reflecting a part of an optical pulse propagating in the optical waveguide or taps for extracting a part of the optical pulse as a transmitted wave are provided at equal intervals; c) The delay time of an optical pulse propagating between the reflective media (tap) by means such as changing the phase velocity of light propagating between the reflective media or taps, or changing the effective waveguide length ( A) a plurality of phase changing means for changing the phase), d) further comprising an optical pulse generating means, and in the case of a reflection type, a reflected signal reflected back from the optical waveguide. Has a light pulse separating means for separating the (light pulse) only,
In the case of a transmission system using a tap, the optical transmission system has an output optical waveguide for guiding an optical pulse extracted by the tap. E) The plurality of reflected signal sequences (optical pulses) or the transmitted signal sequence extracted from the tap. (Light pulse) having an information detecting means for detecting a phase difference between pulses.

<Operation> The basic operation of the optical storage device of the present invention is as follows.

B) In the device of the present invention, the relative value of the phase difference between the reflection media (or taps) is used as stored information. The operation of writing information is performed by changing the relative delay time (phase) of light propagating between a plurality of reflection media (or taps) by a phase changing means.

B) The operation of reading information is performed by introducing a pulse into the optical waveguide, causing the pulse to propagate through the optical waveguide, and determining the relative phase between the reflected signal trains reflected by the reflection medium and returning (or the relative phase of the transmitted signal train extracted at the tap). Phase).

As is apparent from the above, the device of the present invention does not require a means for performing mechanical scanning as in the conventional example, and the information read access is performed by introducing and propagating an optical signal into the optical waveguide,
The time is extremely high, which is the time that is reflected and detected after re-propagation, or the time that the light propagates through the optical waveguide and is extracted by the tap, that is, the order of the propagation delay time of light in the waveguide.

As described above, the device of the present invention is different from the conventional device in both the structure, operation principle, and obtained characteristics.

As a device that can be simply considered based on the same device configuration as that of the present invention, there is a method of detecting an absolute phase in each bit of a signal to be reflected or tapped.
However, in this case, when a large storage capacity is required, it is necessary to guarantee an absolute phase difference over the entire long optical waveguide. This requires a very stable coherent light source. Also, if there is a slight expansion and contraction of the optical waveguide due to a temperature change or vibration, a large phase change appears at a bit position far from the optical pulse input end, and a malfunction easily occurs. For this reason, such devices are not feasible in the state of the art.

In the present invention, this problem is solved by comparing the relative phases of adjacent signals. That is, in the configuration of the present invention, even a slight change in oscillation frequency or expansion and contraction of the fiber requires only a small phase change between adjacent signals, so that stable operation is compensated.

<Embodiment> FIG. 1 shows an embodiment of a reflection type optical storage device. In the figure, 1 is an optical pulse generator, 2 is an optical waveguide which is an information storage area, R ₀ , R ₁ , R ₂ ,... R _n (arbitrary ones are indicated by “R _i ”) are reflection media, E ₁ , E ₂ , ... E _n (arbitrary "E _i "
) Is an electrode, C is a directional coupler, D is an information detector,
I is a Mach-Zehnder interferometer, and t ₁ , t ₂ ,..., T _n (arbitrary ones are indicated by “t _i ”) are signal input terminals of stored signals to each address. Then, although not shown, the phase change means P ₁ , P ₂ ,... P _n (arbitrary ones are indicated by “P _i ”) by the electrodes, the pair of reflection media, and the optical waveguides between the pair of reflection media. Is configured.

As can be seen from Figure 1, the apparatus of the present invention are arranged at equal intervals reflective medium R _i and each reflective medium R _i phase change means between
It has an optical waveguide 2 in which P _i is arranged, and a pulse generator 1 for introducing a short optical pulse is connected thereto.
Has a structure in which a directional coupler C for separating and outputting a reflected signal (reflected light pulse) is installed, and the output signal is connected to an information detector D so as to be introduced.

The optical pulse generator 1 includes, for example, a mode-locked laser,
Any device that can generate a short optical pulse, such as a semiconductor laser having a gain switch or a device using an optical gate, may be used without any particular limitation.

Optical waveguide 2, for example an optical fiber made of a dielectric guide (planar optical waveguide), what kind of, as long as it can allowed to propagate light pulse signal to the distortion without and low loss at the portion except for the reflective medium R _i May be used, and are not particularly limited here. However, here, the embodiment will be described assuming a dielectric or semiconductor optical waveguide whose propagation constant can be controlled by applying an electric field.

In principle, the reflection medium R _i may be any material as long as it is slightly different from other parts in the optical waveguide 2 in terms of physical structure or electro-optical characteristics. For example, in the case of an optical fiber, it can be realized as a mechanical connection point or a portion where stress such as bending is locally applied. In the case of a guide using a dielectric optical waveguide or a semiconductor optical waveguide obtained by diffusing titanium into lithium niobate, it may be realized as a portion where the size or refractive index of the guide is locally changed.

Phase change means P _i may be any kind of long as it can change the phase of an optical signal propagating through the optical waveguide 2. For example, when the optical waveguide 2 is made of a piezoelectric material, it can be realized by applying an electric field, and in the case of a magnetostrictive material, by applying a ground to change the optical waveguide length.
In the case of the optical waveguide 2 having an electro-optic effect, it can also be realized by modulating the propagation speed of light by a change in the refractive index due to the application of an electric field. If the writing speed does not matter, a thermal method may be used.

The information detector D may be of any type as long as it can detect the relative phase difference between adjacent light pulses that have been reflected back. Here, an embodiment will be described for a case where a Mach-Zehnder interferometer I is used.

 The operation as a memory is as follows.

: Write Operation: Address may be considered to correspond to the optical waveguide 2 in each section between locations namely reflective medium R _i of the phase changing means P _i, the relative position of the optical signal in the optical waveguide 2 in each section The memory operation is performed by giving information to the phase difference. Writing information
A signal is introduced through the input terminals t ₁ , t ₂ ,... T _n , and a voltage is applied to the electrode E _i of the phase changing means P _i (i.e., the i-th one) in each section corresponding to the address. This is performed by giving information to be stored such that a change occurs in the relative phase difference of the optical signal propagating therethrough.

In this embodiment, the phase between adjacent bits corresponds to information 1 "(or 0"), and the phase between adjacent bits corresponds to the information 0 "(or 1"). so as to apply a voltage to the electrode E _i. Since for example, as shown in Figure 2, assuming that stores "1100101011 ..." as information, "applying a corresponding voltage to the electrodes E _i, the second bit is 1" 1 of 1 bit is no voltage is applied to the electrode E ₂ for causing a phase change. 3,4 bit is 0 "because the same as the electrode E ₂ so as not to cause a phase change, i.e. no voltage is applied to the electrode E _3, E _4.
5th bit 1 "electrode E ₅ because it is the cause phase changes depending on the state or voltage application different from the state of the electrode E _4.
6 bit 0 "electrodes E ₆ because it is may be determined the state of the phase change means so that the electrodes E ₅ identical or voltage applied.

As described in the description of the structure, the optical waveguide 2 has, for example, an electro-optic effect. Therefore, the propagation constants in the optical waveguide within the second optical signal is changed by the electric field, the relative phase of the optical signal propagating between the reflective medium R _i than when not applied the result field (delay amount) Changes. By appropriately selecting the intensity of the dimensional electric field of the element and the wavelength of light, this variation can be reduced to π / 2.
With this setting, the phase of the reflected wave that is reflected after passing through the section i and then returns again in the incident direction through i is inverted by the application of the electric field. Meanwhile, equal distance between the reflecting medium R _i adjacent to each other, when no voltage is applied to the electrode E _i, the phase change amount of the light propagating through each section is equal. Therefore, the phase difference between the two optical signals reflected by the reflection media R _i and R _i-1 when no voltage is applied to a certain interval i and the preceding interval i−1 is 2
Assuming that 2nπ is set to be an integral multiple of π (this assumption is not necessarily a necessary condition as described later), if it is assumed that voltage is applied only to section i, the distance between the two reflected waves Has 2nπ + π (or 2nπ−π;
The sign may be either. If you store a lot of information,
From the viewpoint of eliminating the deviation of the pulse position, it is rather preferable to use both of them). This phase difference becomes the write information. The above operation corresponds to writing.

: Read operation: In the information read operation, the pulse generator 1 generates an optical pulse at the read timing. The light pulse passes through the directional coupler C and propagates to the optical waveguide 2, and the reflection medium R _i
Reach Although most of the energy of the optical pulse signal propagates as it is in the reflection medium R _{i having} a small reflection coefficient,
Part is reflected and returns to the input end. At this time, depending on the structure of the reflection medium R _i and the attitude of the propagating light, there is a slight disturbance of the waveform, but the incident light pulse is divided into two while almost maintaining its shape, one goes straight as it is, and the other Will propagate in the opposite direction. Light pulses traveling forward passes through the sequentially reflected medium R _i, to generate the reflected wave in each reflector medium R _i. As a result, the wave that is reflected and recedes is the reflection medium R _i
, The propagation path is changed in the directional coupler C, and the information detector D is changed to a signal train in which the relative phase difference between adjacent reflected waves is reflected as information. Be guided. The information detector D consists of circuit for cutting out signals to the interferometer I and a predetermined timing of the Mach-Zehnder type interferometer I is such that the phase difference of light passing through the two branches b ₁ and b ₂ is 2nπ (That is, so as to be equal to the phase difference of the reflected wave in each section when no voltage is applied). In this case, in the interferometer I, the incident reflected wave train is split into two branches and propagates, and interferes with a phase difference of 2πn, that is, reflected waves between adjacent sections. Therefore, when the phase difference between adjacent reflected waves is 2πn (when no voltage is applied to the control means), an optical signal having a large amplitude superimposed on the output end is
When the phase difference is an odd multiple of π, the phase difference is canceled and no optical signal is obtained. That is, the stored information written in the form of applying a voltage by the phase changing means in the writing operation could be converted into the form of the presence or absence of an optical pulse. A predetermined time delay from the trigger pulse, that is, τ0 + nτ (where τ0 is a constant value,
n: integer, τ = 2L / c, L: address position interval, c: propagation speed of light in the waveguide), the reading operation is performed by detecting whether or not an optical pulse exists. .

In this operation, the read time is substantially the time required for the light pulse to reciprocate from the input end to the farthest reflective medium. For example, assuming that a femtosecond pulse is used as an input signal pulse and the position interval is about 10 μm, τ =
It can be about 0.1ps. If the storage capacity is 1Kb, the access time will be as fast as 100ps.

As can be seen from the above operation, in the present invention, the write signal and the read signal are not the same, and in consideration of the fact that the change point is detected when there is a signal sequence, the input pattern is modified in advance to obtain the initial signal sequence at the time of read. This is the embodiment described above. However, the present invention is not limited to such a method. Conversely, it is also possible to apply a method in which a signal sequence is directly applied to the control means, converted after detection, and an original pattern is obtained. Further, it is possible to use the input signal and the output signal without conversion, that is, to use the input signal and the output signal as a circuit having a function of detecting only a change point of the signal.

For convenience of explanation, the adjacent reflection media R _{i of} the above embodiment are used.
The phase difference of the reflected wave from the light source is set to 2πn, but it is needless to say that the phase difference may be the same as the branch difference of the interferometer.

In the above embodiment, the storage device is configured by associating the phases 0 and π with the information. However, by making the voltage applied to the phase control means a continuous amount, the phase difference is associated as an analog amount from 0 to π. It is also possible. In this case, it can be used as an analog memory.

At the time of reading, the signal is detected by comparing only the phases of adjacent pulses. However, the signal detection may be performed by comparing signals separated by one bit or more, or by comparing the phases of a plurality of signals.

FIG. 3 shows an embodiment of an optical storage device of the present invention of a transmission type. In the figure, 1 is an optical pulse generator, 2 is an optical waveguide which is an information storage area, T ₁ , T ₂ ,... _TN (arbitrary ones are indicated by “T _i ”) are taps, and 3 is a tap. The output waveguides shown in FIG.
This is the same as the embodiment in the figure.

As can be seen from the figure, the present embodiment has an optical waveguide 2 in which taps T _i are arranged at equal intervals and a phase changing means P _i is arranged between each tap, and an optical pulse generator 1 for introducing a short optical pulse into the optical waveguide 2. connect, has a connection structure to introduce the output signals from each tap T _i to information detector D. That is, the signal extracting means in the embodiment of FIG. 1 is not a reflection but a transmission signal to a tap.

The tap T _i may be constituted by an optical waveguide having a weak coupling with a wave propagating in the optical waveguide 2. For example, in the case of an optical fiber, it can be realized by coupling the light leaking from a portion subjected to stress due to local bending or the like with the tap output optical waveguide 3. In the case of a dielectric waveguide or a semiconductor guide obtained by diffusing titanium into lithium niobate, an optical waveguide for tapping can be provided adjacent to the optical waveguide 2 to realize the coupling mode of the two optical waveguides. I just need. The coupling coefficient of the tap is not particularly limited, but is preferably as small as possible when a large storage capacity is required.

Compared to the first embodiment, the memory uses a transmitted wave instead of reflection, so that the basic operation is exactly the same, although the signal timing relationship is slightly different. That is, in the case of the reflection structure, the time required to reciprocate between the reflection media corresponds to approximately one bit length, whereas in this case, the length of one tap section of the optical waveguide 2 and the optical waveguide 3 is equal to the length of one bit. The time required to propagate the distance corresponding to the difference corresponds to approximately one bit length. Although there is a difference between a signal train at the time of reading and a reflected wave or a transmitted wave, this is not a difference in the operation principle as a memory. The reflected wave in the first embodiment is regarded as a transmitted wave, and the detailed description of the operation is omitted here.

In the case of this embodiment, the maximum read time substantially corresponds to the time required to propagate through the optical waveguide 2 to the end. Therefore, also in this embodiment, the same high-speed reading operation as in the embodiment of FIG. 1 can be performed.

It goes without saying that the same change as described in the first embodiment can be made in the transmission type.

<Effect of the Invention> As described above, according to the device of the present invention, it is possible to realize an optical storage device that does not require mechanical scanning as in the conventional example and can perform extremely high-speed access according to the light propagation speed. .

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an optical storage device according to a first embodiment of the present invention, FIG. 2 is an explanatory diagram showing an example of a storage state of the first embodiment, and FIG. FIG. 2 is a configuration diagram illustrating an optical storage device according to an example of the present invention. In the drawing, 1 is an optical pulse generator, 2 is an optical waveguide, R ₁ , R ₂ ,... R _n are reflection media, P ₁ , P ₂ ,... P _n are phase changing means, E ₁ , E ₂ ,. _n
Is an electrode, C is a directional coupler, D is an information detector, t ₁ , t ₂ ,
... t _n is the signal input _{terminal, T 1, T 2, ...} T n is tapped.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G11C 13/04 G11C 11/42 G02F 3/02 G11C 19/30 G11C 21/00

Claims (2)

(57) [Claims] An optical waveguide for propagating an optical pulse; an optical pulse generating means for inputting an optical pulse to one end of the optical waveguide; and an optical waveguide provided at intervals along a propagation direction of the optical pulse. A plurality of reflection media for reflecting a part of an optical pulse propagating in a wave path; a plurality of phase changing means for changing a phase of an optical pulse propagating in the optical waveguide between the respective reflection media; Optical pulse separating means for separating and extracting the optical pulse reflected by the optical pulse and returning to the direction of the optical pulse generating means, and a phase difference between each optical pulse reflected by each reflecting medium and extracted by the optical pulse separating means. An optical storage device, comprising: information detection means for detecting information as information. An optical waveguide for propagating an optical pulse; an optical pulse generating means for inputting the optical pulse to one end of the optical waveguide; and an optical waveguide provided at intervals along a propagation direction of the optical pulse. A plurality of taps for extracting a part of the optical pulse propagating in the wave path; a plurality of phase changing means for changing the phase of the optical pulse propagating in the optical waveguide between each tap; and a plurality of taps extracted from each tap. An output optical waveguide for guiding an optical pulse, and information detecting means for detecting a phase difference between each optical pulse taken out at each tap and sent through the output optical waveguide as information, and Optical storage device.